There are known the heat-developable light-sensitive materials capable of easily and rapidly providing a dye image in a thermal dry process, and such light-sensitive materials and image forming methods are described in Japanese Patent Examined Publication Nos. 4921/1968 and 4924/1968; the `Silver Salt Photograph` section in `Fundmental Photography` (Corona, 979) pp.553-555; and Research Disclosure 17029, pp. 9-15, June 1978.
Heat-developable light-sensitive materials are classified into a black-and-white type and a color type. Recently, an emphasis is put on a development of the heat-developable color light-sensitive materials capable of forming color images with various dye donors.
There are various types of heat-developable color light-sensitive materials. One example is a light-sensitive material in which a color image is formed by transferring a diffusible dye that is released or formed by heat development (hereinafter called a dye transfer type). Though this type of material requires an image receiving member for a transferred image, it is excellent in an image stability a sharpness and an easy and rapid processing. Such transfer type materials and the image forming methods therefor are described in Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) Nos. 12431/1975, 159159/1984, 181345/1984, 229556/1984, 2950/1985, 52643/1986, 61158/1986, 61157/1986, 180550/1984, 132952/1986 and 139842/1986; and U.S. Pat. Nos. 4,595,652, 4,590,154 and 4,584,267.
The above conventional heat-developable light-sensitive material is liable to have a bad image discrimination. The solution for this problem is proposed in Japanese Patent O.P.I. Publication Nos. 301036/1988 and 301037/1988, in which there is disclosed an improvement of a developability of the light-sensitive material by adding a heterocyclic compound having a mercapto group. However, the image discrimination can not be improved sufficiently even with this technique, and there is left a problem of preservability deterioration attributable to the mercapto compound. In order to prevent the preservability deterioration, it has been required that the light-sensitive materials contain a development accelerator in the form of a stable precursor having a masked mercapto group.
Japanese Patent O.P.I. Publication No. 159642/1986 discloses a heat-developable light-sensitive material containing a coupler which releases a development accelerator by reacting with an oxidation product of a developing agent. This technique, however, can not provide a sufficient image density, and it is necessary to further improve the image discrimination by adding a stable development accelerater to a heat-developable color light-sensitive material.
Further, the conventional heat-developable color light-sensitive materials are liable to have a larger fluctuation of a minimum density of an image against fluctuations of the developing conditions, and therefore there has been a demand for a heat-developable color light-sensitive material having a further more excellent preservability.